The impact of intercritical annealing in conjunction with warm deformation process on microstructure, mechanical properties and TRIP effect in medium-Mn TRIP steels

We propose here for the first time, warm deformation processing involving intercritical annealing of Fe-8.5Mn-0.2C-3.0Al medium-Mn TRIP steels to obtain outstanding tensile strength-ductility combination. A critical analysis of results demonstrated that warm deformation is characterized by excellent mechanical properties. The multiphase microstructure of warm-deformed steels was relatively ultrafine. The PSE (product of tensile strength and total elongation) approached 27–53 GPa%, and the value was significantly high compared to the previously reported hot-deformed boron steels and medium manganese steels. When the deformation temperature decreased from 770 to 710 °C, the austenite volume fraction increased from 35.6% to 53.1%, resulting in high tensile ductility (up to 53.6%). The difference in intercritical annealing temperature had a significant effect on the tensile deformation behavior of warm-deformed steels: at 710 °C, the tensile deformation behavior exhibited discontinuous TRIP effect and Lüders band propagation (LBP). While at 730 °C, only discontinuous TRIP effect occurred in the whole plastic deformation region. The ductility of warm-deformed parts heated at 690 °C was mainly related to the deformation of δ-ferrite. The increase in warm deformation temperature led to significant changes in the characteristics and mode of fracture.